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Intermediate Guide

4.   Imaging an asteroid or comet

Plotting the asteroid's or comet's path on your charts and noting its brightness are great ways of learning about them. And, until the pencil marks fade you'll be able to remember where you saw them. A more permanent way of recording what you see is to sketch or photograph your view.

comet drawing by Paul Medcraft
Fig 1: Sketch of C2000 WM1 (LINEAR) by Paul Medcraft

Way back in Section 7 in the Beginner's Guide, there was a template for an observing log. And a sketching tutorial to show you how to sketch. Most logs will include some kind of sketch. Hand drawings can be quite simple since we're not all artists, but some observers can make some absolutely stunning drawings. The nice thing about sketching is that it forces you to sit and watch your target for a long time as you draw the details. The longer you observe, the more details you'll see. Many times, people have glanced into a telescope at an object and come away disappointed primarily because they looked at it for less than a minute. Can you remember what someone looks like if you only look at them for a minute? No (well maybe if you have a photographic memory), so take your time. When observing comets, look for jets in the coma, knots of debris in the tails, the shape, color and orientation of the tails,... the comet may have recently broken apart so you might be able to see the many pieces, there's all kinds of things to notice if you just look! Observing details for asteroids is a bit tougher since they appear starlike, but you might be able to see color and brightness differences as the asteroid rotates.

More on...

Not all of us are artists but we still want to record and remember what we saw, so photographing the comet might be the way to go. Because comets do vary in brightness and some come closer to earth than others, there are many techniques for photographing them. There isn't enough space to go into all that detail, so I'm going to concentrate on how to photograph the target of the Deep Impact mission, comet 9P/Tempel 1.

In the weeks before impact, Tempel 1 will be a faint fuzzball. A camera on a tripod will not likely 'see' it. But if you are in a really dark location and have a big lens, it wouldn't hurt to try! The next 'simplest' method is to have a camera with a telephoto lens piggybacked on a telescope (or barndoor tracker) taking advantage of the tracking capability. This allows one to take a longer exposure thereby seeing dimmer objects. It does require that the telescope be carefully polar aligned. And again, clear, dark skies will give better results.

The method that will likely yield the 'best' results will be to mount your SLR camera sans lens to your telescope (prime-focus). Essentially the telescope becomes a giant telephoto lens for the camera. Polar alignment of the telescope is extremely critical, but it will also help to have some type of guiding system in place. The comet is also moving through space albeit relatively slowly, but over a long enough exposure, your stars might be pinpoints, but the comet might have smeared. By guiding on the comet, you'll get a sharper image of the comet, and in this case, it will be okay to have startrails. What film to use? Well, every astrophotographer will swear by one or another -- I personally prefer slide films up to 400 ASA. And now there are digital SLRs that don't use film, but be sure to have your batteries charged up! There is plenty of time to practice and experiment since the comet is currently (October 2004) visible in the morning sky and rising earlier and earlier so that by the spring, it is up all night long.

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Updated: 30-Jul-2013